The present invention relates to a method for producing the active elements in liquid crystal displays. More particularly, it relates to a method for producing lateral Metal-Insulator-Metal (MIM) devices for liquid crystal displays.
It is known to incorporate MIMs in liquid crystal displays. FIGS. 8 and 9 show the structure of MIM devices.
FIG. 9, shows an MIM device comprising a transparent substrate 27, a first conductor 21 having side 22, a barrier layer 23, a second conductor 25 and a pixel electrode 26. The electrical resistance of the thick portion of barrier layer 23 has been set high so that it does not function as an MIM device. First conductor 21, second conductor 25 and thin portion of barrier layer 23 along first conductor side 22 form a non-linear MIM device.
By structuring the device in this manner, the surface area of the MIM device can be quite small. This technology is effective in achieving high density in liquid crystal displays.
However, use of MIMs has been hampered by manufacturing problems which result in defective liquid crystal display (LCD) panels. In particular, the problem is that yield is extremely low, making the device impractical. FIGS. 10(a)-(e) and 11(a)-(e) illustrate the MIM manufacturing process flow of the prior art.
FIGS. 10(a)-(e) shows the production steps of the prior used to fabricate a lateral MIM device.
In step (a), the film for first conductor 21 is formed on transparent substrate 27. This includes the formation of a transparent base film.
In step (b), barrier layer 23 is formed on top of first conductor 21.
In step (c), patterning takes place through the simultaneous photo-etching of first conductor 21 and barrier layer 23. Barrier layer 23 will subsequently act as a first thick insulator when the device is fully fabricated.
In step (d), second insulator film 24 is formed on the side of first conductor 21, which was obtained via step (c).
In step (e), second conductor 25 and pixel electrode 26 are each formed and patterned.
The lateral MIM device is formed by first conductor 21, second insulator 24 and second conductor 25.
Well-known conductors used for MIM devices are Ta, Al, Au, ITO, NiCr+Au and ITO+Cr. Well-known insulators that are used for the second insulator are TaOx, SiOx, SiN.sub.x, SiOxN.sub.y, TaN.sub.x and ZnOx. These second insulators are formed through thermal oxidation or anodic oxidation or through a process such as sputtering.
For the barrier layer, in addition to the inorganic materials used for the second insulator above, organic material such as polyimide may also be used.
The best known structure for an MIM device uses Ta (tantalum) for first conductor 21, TaOx (tantalum oxide) for second insulator 24, and Cr (chromium) for second conductor 25, yielding a Ta/TaOx/Cr structure.
FIGS. 11(a)-(e) correspond to FIGS. 10(a)-(e). In general, the barrier layer 23, whether made from a nitride or an oxide, will have a lower etch rate than first conductor 21. TaOx etches at about half the rate of Tantalum. The etch rate difference between Ta and TaN.sub.x is even greater. As a result, as shown in FIG. 11(c), first conductor 21 takes on an undercut shape.
In subsequent step (d), second insulator 24 (TaOx) is formed.
In step (e), second conductor 25 (Cr) and pixel electrode 26 are formed. However, when forming second conductor 25, there is the drawback that second conductor 25 will be subject to breakage because of the overhang of second insulator 24. Moreover, even if there is no breakage, because the shape, or taper, of the first conductor sides cannot be controlled, there is the problem of not being able to control the surface area of the lateral MIM device.
Failure to control the MIM surface area means that the display condition of the LCD pixel cannot be controlled, and this is a fatal defect.
What is desirable is a process for reliably and uniformly producing a plurality of lateral MIM structures on a substrate such that high yields and predictable performance can be achieved in LCD panels.